1. The Field of the Invention
The present invention relates to the field of shielding electromagnetic interference through the use of electrically conductive shells or casings.
2. The Relevant Technology
As the trend for electronics and electrical devices continue to move towards miniaturization, the effects of electromagnetic interference due to electromagnetic radiation continue to become more profound. This interference can be highly disruptive to an integrated circuit as it can distort the signals carried within the circuit, thereby adversely impacting its performance. This problem is exacerbated in high frequency applications such as those in the field of communications that are becoming far more prevalent today.
In general, steps taken to address this problem are usually applied directly on the circuitry through means such as the use of bypass capacitors, the slowing of rise and fall times as well as minimizing noise from ground and supply lines. However, in today's technology where many high-speed components are often clustered together in small, densely packed applications, these solutions remain inferior. Another solution is the use of electromagnetic shielding. An electromagnetic shield can be employed to contain electromagnetic radiation emitted from an electromagnetic source, as well as to protect sensitive circuitry by shielding it from electromagnetic interference.
In order to reduce the effects of electromagnetic interference, the shields are usually able to absorb and/or reflect electromagnetic interference energy, and are often presented in the form of enclosures comprising of electrically grounded conductive walls that act as a barrier for electromagnetic interference and electromagnetic radiation. These walls are are usually made of metal or conductively painted plastic, and can sometimes involve the use of conductive gaskets in the sealing of the enclosure. Since there is a need for circuitry to be accessible, it is advantageous for the enclosure to be of a semi-temporal nature, and this is often implemented through the use of access panels, doors and the like. However, the use of such accesses also leads to diminished electromagnetic shielding as they invariably lead to seams between the removable accesses and the enclosure, which provide an opening in which electromagnetic radiation may enter into or escape from the enclosure. These gaps can also disrupt the ground conduction path and in the case of a Faraday cage, inhibit the electromagnetic shielding of the cage through the electrical discontinuities caused by the gaps between the electrically conductive walls of the enclosure. Much of the prior art is hence centered on the minimization of these openings and the resulting electromagnetic radiation and/or electromagnetic interference.
One way of doing so is through the use of gaskets. However, the use of gaskets can be costly as they are required to be wear resistant and capable of withstanding repeated compression and relaxation cycles, and adds to the manufacturing cost by increasing the inventory management and product cycle time. The use of gaskets may also necessitate their replacement occasionally due to corrosion and/or the loss of compressibility, which further adds to the cost.
Other prior art solutions attempt to minimize the gap in the enclosures by utilizing mechanical means to secure the access panels and the like to achieve a seamless enclosure while maintaining good electrical conductivity throughout the enclosure. However, the nature of such designs is such that there still remain seams between the joints regardless of the mechanical pressure applied in joining these parts due to the unavoidable tolerances in the manufacturing process, causing imperfections in the fabricated parts and surfaces. This results in less than ideal joints between mating surfaces.
One way of improving the electromagnetic shielding of an enclosure without the weaknesses of the above solutions is by designing a step along its walls, as illustrated cross-sectionally in FIG. 1. This step reduces the electromagnetic radiation by introducing a perpendicular turn (104) at the interface of the top (102) and bottom (103) enclosure walls, which weakens the energy of the transmitter radiation by having the geometrical interference absorb it. This solution also has its disadvantages. First, the design is susceptible to electromagnetic radiation polarized in a direction parallel to the opening of the steps. Second, due to tolerances allowed in the fabrication process, any rounding of the edges of the steps would allow electromagnetic radiation (101) to pass through the gaps by bending past the edges. Third, design constraints, particularly in communications modules, often limits the thickness of the walls to be used for the shielding enclosure. In thin walls, the geometrical interference provided by the steps may not be sufficient to attenuate the electromagnetic radiation to minimize the interference caused.
The use of an electromagnetic interference paste to seal the gaps might be sufficient to address the weaknesses inherent in the respective prior art. There are several disadvantages in this. First, the use of electromagnetic interference paste increases cost, both in the sale of the paste and the processing cost involved in dispensing the paste onto the interfacing joints. Second, the use of electromagnetic interference paste necessitates the need for curing, which requires additional time and effort in the application of heat and increases the product cycle time further.